The present invention relates to a particle concentration device, and in particular to a particle concentration device suitable for concentrating sample particles in a solution, using ultrasound.
It has been known since the 19th century that particles in a fluid can be trapped without contact by irradiating the particles with ultrasound. Concerning acoustic radiation force that particles receive when the acoustic radiation force acts on the particles, for example, in Acoust. Soc. Am. 89(1991) pp. 2140-2143, J. Wu, J. reported that he succeeded in trapping polystyrene spheres of 270 .mu.m diameter at the focal point of focal ultrasound. As regards the principle that particles are trapped by acoustic radiation force, in Acoustica 5 (1955) pp. 167-178, K. Yosioka and Y. Kawasima reported that they calculated the intensity, in an ideal fluid, of acoustic radiation force which particles receive in a standing wave and a traveling wave, and the acoustic radiation force which the particles levitated in the standing wave receive is in proportion to the volume of the particles and the frequency of the ultrasound forming the standing wave. Furthermore, Japanese Patent laid open No. 7-47259, proposed by the present inventors, discloses a manner of introducing ultrasound into a tube in which a fluid is allowed to flow so as to focus particles continuously within some area, or a method for collecting the focused particles.
It has been known heretofore that as the frequency of ultrasound used in a standing wave is gradually changed, the position of nodes of the standing wave changes accordingly and further particles also move accordingly. In J. Acoust. Soc. Am. 91(1992), pp. 3152-3156, T. L. Tolt et al., reported a means for moving and concentrating particles trapped in nodes of a standing wave actually by sweeping, upwards and downwards, the frequency of ultrasound introduced into a fluid wherein the particles are dispersed. Furthermore, U.S. Pat. No. 5,225,089 by E. Benes et al. discloses a means for concentrating particles by raising the frequency of ultrasound radiated from an ultrasound source arranged in a channel.
Moreover, it has also been known heretofore that the position of nodes of a generated standing wave can be controlled by controlling the phases of ultrasounds radiated from a pair of opposite ultrasound vibrators for generating a standing wave. U.S. Pat. No. 4,743,361 by C. J. Schram discloses a means of applying this technique actually to measure physical properties of particles by observing how much the particles follow the movement of the position of nodes of a standing wave. It has also been reported that when ultrasound having slightly different frequencies are radiated oppositely, the position of nodes of a generated standing wave advances by the slight difference between the frequencies.
Additionally, it has been known that the generation of cavitation which may damage a sample can be suppressed by raising the frequency of used ultrasound. It has also been known that when, for example, ultrasounds having the same density are introduced, the sound pressure peak p.sub.c of its cavitation threshold is in proportion to the frequency f of the incident ultrasounds. As reported in, for example, Acustica 24 (1971) pp. 191-196 by G. Iernetti, cavitation can be prevented only by raising the frequency of ultrasound. Actually, in the step of washing silicon wafers by irradiation with ultrasound in pure water in the process of producing semiconductors, the frequency of used ultrasound is within the MHz range in order to prevent the surfaces of the silicon wafers from being damaged by cavitation based on the ultrasound.